1. Field of the Invention
The present invention relates to bearings suitable for use in automobiles, ships, agricultural machines and the like and, more particularly, to a multi-layer type sliding bearing of aluminum alloy which excels in the anti-seizure property, the anti-fatigue property and the anti-fretting property.
2. Description of the Related Art
It has heretofore been common practice that sliding bearings of aluminum alloy are used in gasoline engines or engines for small automobiles with the sliding-bearing surfaces being coated with no overlay. On the other hand, bearings of aluminum alloy which have an Ni-plating intermediate layer and an overlay consisting essentially of a Pb alloy have been employed in the field of sliding bearings which are primarily used with special types of engines such as high-load engines or medium-speed diesel engines. Another example of the prior art is known in which Sn or an Sn alloy is applied as an overlay to a bearing of Cu-Pb alloy or Pb-bronze alloy which is exclusively intended for engines for use in specific types of medium- or low-speed ship. This prior art makes use of the properties of Sn alloys having corrosion resistance and abrasion resistance which are high compared to those of Pb alloys. The art of providing an Sn overlay on an aluminum alloy is shown in Japanese Patent Unexamined Publication No. 61-153286, which discloses a bearing member in which a bearing layer of aluminum alloy is directly plated with Sn without an Ni intermediate layer.
The opportunities for employing gasoline engines or diesel engines for use in small automobiles such as passenger cars in the range of high-speed rotation have been increasing particularly recently. In the situation, it has been desired to improve to a great extent the seizure resistance, abrasion resistance, fatigue resistance, corrosion resistance and fretting resistance of engine bearings.
As is known, a conventional type of bearing of Cu-Pb or Pb-bronze alloy provided with an Pb-alloy overlay is disadvantageous in terms of corrosion resistance. Moreover, it has recently been noted that the properties of lubricating oil are deteriorated rapidly and seriously during use thereof, particularly due to an extension in the life of engine bearings and a rise in oil temperature as a result of an increase in the rotational speed of engines. In particular, an organic acid resulting from the deteriorations of lubricating oil is apt to corrode the Cu-Pb alloy or the Cu-bronze alloy to a remarkable extent. In addition, in order to achieve a reduction in the manufacturing cost of engines, there are many cases where crankshafts are made of ductile cast iron. However, the use of ductile cast iron often causes rapid abrasion or rapid seizure in the above-described conventional three-layer type bearings.
In view of the problems, bearings of aluminum alloy have recently been being employed in place of such Cu-Pb type three-layer bearings. Aluminum alloys are generally compatible in nature with shafts of ductile iron and have a satisfactory corrosion-resistant property. However, if a bearing made of such an aluminum alloy is employed without an overlay, partial striking or misalignment is apt to occur. Moreover, since dust is not easily separated from aluminum alloys, the phenomenon of seizure may take place early in the service life of the bearing if the application conditions are not satisfactory. If an overlay consisting essentially of a conventional Cu alloy is provided on such a bearing of aluminum alloy as shown in Japanese Patent Unexamined Publication No. 62-110021, it follows that the problems of corrosion resistance and abrasion resistance are again encountered. To overcome these problems, Japanese Patent Unexamined Publication No. 60-36641 discloses the art of utilizing, instead of an Pb alloy, an Sn alloy as an overlay. Japanese Patent Unexamined No. 61-153286 discloses a bearing member in which a bearing layer of aluminum alloy is directly plated with Sn without an Ni intermediate layer.
These conventional methods utilizing overlays, however, have the following problems. In a case where an overlay is provided on an aluminum alloy by electroplating, it is customary to provide an intermediate layer of Ni, Co, Fe or the like in order to ensure the adhesion of the overlay. However, such an intermediate layer is extremely hard and, when the overlay wears out so that the surface of the intermediate layer is exposed, the intermediate layer comes into frictional contact with the shaft to cause seizure or galling. Various disadvantages incurred by the provision of the intermediate layer have been pointed out, particularly in recent years. Although there exists a method of directly providing an overlay without forming the intermediate layer, the bonding strength of the overlay which is realized by this method is extremely low compared to the bonding strength of an overlay combined with an intermediate layer (in the prior art, typically having 0.5 to 3.0 microns in thickness). This type of overlay which is directly provided has a good initial compatibility but inferior durability and there is such disadvantage that laminer abrasion or the exfoliation of the overlay is apt to occur. For this reason, this overlay has been merely provided as a sacrificing layer.
Moreover, the recent trend toward reductions in the size and weight of engines has lead to decrease in rigidity of the housing of an engine, so that the phenomenon of fretting (the phenomenon of abrasion occurring between two contacting surfaces which are exposed to a relative motion of small amplitude) occurring on the rear face of the bearing has become a serious problem to be improved.